Hi Trasgu,
In order to determine the cause of the slowness, please provide us with the following information.
> Did you made any changes in the peer prior to this issue or any additional traffic introduced into the tunnel?
> What is the Encryption method used for VPN? Advanced encryption can introduce latency is a network due to heavy processing.
> One of the major reason for latency can be fragmentation, so, please alter the MSS accordingly - https://kb.juniper.net/InfoCenter/index?page=content&id=KB30688&actp=METADATA
Let me know if you face any difficulty.